Results 2 issues of suem

Picked from https://github.com/mwcproject/secp256k1-zkp/pull/5, fixing failed build jobs 37 and 38: https://travis-ci.org/github/mimblewimble/secp256k1-zkp/builds/666720542.

Pick from https://github.com/mwcproject/secp256k1-zkp/pull/4 There are 2 building failure cases in CI for `BUILD=check-java` which is caused by Java version: https://travis-ci.org/github/mimblewimble/secp256k1-zkp/builds/666720542 It's better to remove these Java interface, same as this...